Applicants have developed a composition and method for use in photoluminescent barometry which greatly improves the sensitivity of methods of photoluminescent barometry. Applicants are aware of the following U.S. patents.
U.S. Pat. Nos. 3,230,764; 4,752,115; 3,612,866; 4,789,992; 3,787,874; 4,810,655; 3,890,835; 4,819,658; 4,075,493; 4,895,156; 4,215,275; 5,043,285; 4,560,286; 5,043,286.
The disclosures of the above patents are incorporated by reference herein.
The scientific literature includes: "Luminescent Barometry in Wind Tunnels," Janet Kavandi et al., Rev. Sci. Instrum. 61 (11), November 1990; "A New Optical Pressure Measurement System (OPMS)," Arne Vollan et al., IEEE, 1991, "Spectroscopic Characterization of Complexes of Ruthenium (II) and Iridium (III) with 4,4'-Diphenyl-2,2'-bipyridine and 4,7-Diphenyl-1,10-phenanthroline, R. J. Watts and G. A. Crosby, Journal of the American Chemical Society, 93:13(3184) 1971; "Mechanics of Gas Transport in Poly(1-Trimethylsilyl-1 Propyne)," S. R. Auvil et al., Polymer Preprints, 32:3(380) 1991; "Substituted Propyne Polymers--Part II. Effects of Aging on the Gas Permeability Properties of Poly[1-(trimethylsily) Propyne] for Gas Separation Membranes," Michael Langsam and Lloyd M. Robeson, Polymer Engineering and science, 29:1(44) 1989; "New technique of surface flow visualization based on oxygen quenching of fluorescence," John I. Peterson and Raphael V. Fitzgerald, Rev. Sci. Instrum. 51(5) May 1980 p. 670; "Fiberoptic Oxygen Sensor Based on Fluorescence Quenching and Energy Transfer," Ashutosh Sharma and Otto S. Wolfbeis, Applied Spectroscopy, 42:6(1009) 1988; and "Photophysics and Photochemistry of Oxygen Sensors Based on Luminescent Transition-Metal Complexes," E. R. Carraway and N. J. Demas et al., Anal. Chem., 63(337) 1991. These publications are incorporated by reference herein.
The field of luminescent barometry is new and has been developed in response to the difficulties arising from conventional techniques used to determining pressure distributions over aerodynamic surfaces, such as, over the surfaces of an airfoil. These difficulties include the difficulty and cost of using mechanical or electronic pressure sensors on a surface. Sensors can disrupt the air flow over the surface and provide an erroneous reading. Errors may also arise due to the difficulty of using a sufficient number of sensors in an array to obtain realistic view of the phenomenon occurring. Many of these difficulties are described in detail in U.S. Pat. No. 3,787,874. More particularly, the aerodynamic forces acting on an aircraft model result largely from the distribution of pressure over the model surfaces. It is therefore common practice to measure surface pressures using a pressure model in the wind tunnel and compute the force distributions from the pressure data. Pressure models have hundreds of small (typically 0.02-inch diameter) pressure taps machined into their surfaces. These taps must be drilled precisely normal to the surface, with tight tolerances on location, diameter and even chamfer. In addition, each pressure tap must be connected to a pressure transducer module. There are usually too many taps to leave room for the transducers inside the model and hundred of thin stainless steel tubes must be used to establish connections to externally located transducers. The precision machining and the hand labor of installing the tubes is slow and costly: it usually takes nine months or more and about $1 million to build such a model. Moreover, overall forces and moments cannot always be measured accurately with conventional methods using pressure models, since the tubes connecting the pressure taps to the transducers can disrupt the airflow around the model and create inaccurate measurements of drag and lift.
Luminescent barometry has developed as a result of the discovery that surfaces may be coated with materials which are capable of being excited by light, such as ultraviolet light, and that these excited materials, will emit light which may be used as a measure of pressure on those surfaces. Many of these materials operate by an oxygen quenching phenomenon, that is the oxygen permeating through the binder which is applied to the surface is a function of the air pressure on the surface. The light emanating from the excited surface is an inverse function of the oxygen partial pressure and therefore the emitted light is an indication and measurement which can be correlated back to the pressure on the surface itself.
The techniques of photoluminescent barometry have been described in "A New Optical Pressure Measurement System (OPMS)," Vollan et al, 1991 IEEE and also in "Luminescent Barometry In Wind Tunnels," Kavandi et al., Rev, sci. Instrum., 61 (11) November 1990. As described by Kavandi et al., a platinum complex, platinum octaethylporphyrin (PtOEP) has been shown to be a suitable photoluminescent compound which may be excited by ultraviolet radiation and used to map the pressure differentials across surfaces, such as airfoils. Other platinum complexes, such as platinum etioporphyrin, have been reported as emitters. The technique can be used on models measured in wind tunnels and can provide pressure distribution maps of the surfaces under examination. In addition to platinum octaethylporphyrin (PtOEP), applicants have found two additional classes of materials which are unexpectedly effective as luminescent barometry photo emitters, or active agents. These include ruthenium complexes and combinations of the organic compounds pyrene and perylene. Pyrene and perylene work as emitters when used in combination. Optimally an equimolar mixture is preferred, but it is not critical or essential. Formulations based on the emitter mixture of pyrene and perylene are most useful for a limited amount of time, probably due to one or both of the emitter compounds subliming out of the binder. Low pressures accelerate sublimation which restrict the useful pressure range of pyrene/perylene coatings to above about 2 psia.
Applicants' new materials have the additional unexpected property of being suitable to excitation by visible blue light as well as by ultra violet light and may be excited and measured by known methods. These latter materials may be incorporated in the silicone polymers described by Kavandi et al. However, applicants have discovered that a new group of cross-linked rubbery siloxane polymers and polyacetylene (polypropyne) polymers produce coatings having unexpectedly superior properties as photoluminescent coatings. A variety of these materials are available commercially from suppliers such as General Electric, Dow Chemical Co. and HULS under a variety of trademarks such as GE RTV 108, GE RTV 118, Dow 734, Dow 732, Dow 3140, HULS PS078.9 (solid polypropyne resin), HULS PS079.5 (polypropyne 5% solution) and equivalent materials. While it is not certain why these binder polymers provide such unexpected improvement in sensitivity, it is believed they have an ability to permit more oxygen, and more rapid oxygen absorption, through the surface to provide a quenching effect to the luminescence and thus a more sensitive reading of the pressure on a particular element of the surface being mapped.
The development of a successful pressure sensitive paint formulation requires not only the synthesis of a suitable active ingredient, but also an appropriate binder material. Efficient quenching of the luminescence requires that the molecules of the active ingredient be uniformly dispersed in a highly oxygen-permeable binder. To withstand the forces acting on the pressure sensitive paint coating during wind tunnel testing, the binder must adhere well to the model surface. To avoid disturbing the air flow, the binder must also be thin and smooth. The present invention meets these requirements and produces a pressure sensitive paint of unexpected sensitivity.
A variety of extenders and fillers may be added to the pressure sensitive paint. In particular silica fillers may be used to extend the pressure sensitive paint. Typical materials include fumed silicas of the type available under the trademark CABOSIL from Cabot Corporation. Silica gels may also be used, in particular, the grades known as TLC and HPLC may be used. When using fillers it is preferred to use a fine particle size to avoid imparting excessive surface roughness to a test model, although this is not critical. In addition, we have found that an unexpected interaction occurs when silica gel is used in conjunction with propyne resins. The combination imparts greatly enhanced sensitivity to pressure sensitive paint formulations. Why the increase occurs is not certain, but it is believed that this specific combination provides greatly enhanced permeability to the coating and thus increases the oxygen quenching effect. We believe that the high surface area and the specific surface characteristics of the silica gel may permit the molecules of the active ingredient to remain well-dispersed in an unaggregated state, and thus more accessible to oxygen quenching.
It is thus an object of the invention to produce a pressure sensitive paint, for use in methods of luminescent barometry, having greater sensitivity in mapping pressure distributions over aerodynamic surfaces.
It is further an object of the invention to produce a pressure sensitive paint, for use in methods of luminescent barometry, having greater oxygen permeability through the matrix of the paint.
It is an object of the invention to produce a pressure sensitive paint, for use in methods of luminescent barometry, having novel photoluminescent active agents.
It is an object of the invention to produce a photoluminescent paint, for use in methods of luminescent barometry, using ruthenium complexes as photoluminescent active agents.
It is an object of the invention to produce a photoluminescent paint, for use in methods of luminescent barometry, using pyrene and perylene photoluminescent agents.
It is a further object of the invention to produce a photoluminescent paint, for use in methods of luminescent barometry, which can be activated by visible blue light.
It is an object of the invention to produce an improved photoluminescent paint, for use in methods of luminescent barometry, containing silica gel to enhance the sensitivity of the paint matrix to oxygen quenching.
It is an object of the invention to produce a pressure sensitive paint, for use in methods of luminescent barometry, which has a cross-linked rubbery siloxane resin matrix.
It is a further object of the invention to produce a pressure sensitive paint, for use in methods of luminescent barometry, which uses poly [1-(trimethylsilyl)propyne] as a binder.
These and further objects of the invention will be understood from the following Description of the Preferred Embodiments and the included Figures.